The CEO of UMass Memorial Health Care issued an apology yesterday for the hospital's practice of using professional models as lures to recruit potential bone marrow donors. In a memo to doctors and other members of the Worcester medical institution, John O'Brien called the practice "an error in judgment" and said it had been discontinued. UMass Memorial has used the models, women dressed in short skirts and heels, to work booths at malls and public events in Massachusetts, New Hampshire, and Rhode Island to solicit people to be tested for a potential life-saving bone marrow match. Investigators from Massachusetts and New Hampshire are looking into the practice, as well as whether UMass Memorial charged insurers high rates for testing the DNA samples and misled donors about the cost of DNA testing.